Barry and Honey Sherman are shown in a handout photo from the United Jewish Appeal.

Warning this story contains graphic details that may disturb some readers.

Canadian billionaires Barry Sherman and his wife, Honey, may have been murdered by one or more killers, according to new reports.

Private investigators hired by the Sherman family believe the couple died in a professional killing, the Toronto Star reported.

The theory was corroborated in a CBC News story, which also revealed that there may have been more than one killer involved.

Both stories point to markings on the Shermans' wrists as one of the most critical pieces of evidence in favour of the double homicide theory. The marks suggest that at some point the Shermans had their hands tied, even though no bindings were immediately found.

No drugs in their systems

"There were no bindings found at the scene, but the autopsy, the post-mortem, showed that there were markings on the wrists," Kevin Donovan, the Toronto Star's reporter for the piece, told CTV News. "Police were looking for stuff in the sewers and one could imagine they were looking for something like that, rope or plastic ties that may not have made it all the way to the sewer system."

The couple also tested negative for any drugs that could have played a part in their deaths, according to the Star's story. The "ligature neck compression" that caused their deaths, previously thought to be an indication that the couple were hanged, was the result of leather belts pulled around their necks, the reports say.

The Shermans bodies were found on Dec. 15 by a real estate agent visiting their mansion, which was listed for sale. They were likely killed on Dec. 13, according to the CBC News story, because Honey was still wearing the clothes she had been wearing that day.

'Suspicious' deaths

Toronto police initially ruled the Shermans' deaths as "suspicious" and have not publicly released any updates on the case.

"No one is in a better position to make a determination as to what this is and what this isn't other than the homicide squad," Mark Mendelson, a former Toronto police homicide detective told CTV News. The police will probably release more information when their investigation is finished, he added.

The private investigation's findings contradict an earlier theory that was circulating in the media, reportedly from an anonymous police source, that the Shermans died in a murder-suicide.

The couple's children disputed that theory from the get-go and launched their private investigation soon after the murder-suicide theory was reported by media.

Barry founded Apotex, a Toronto-based pharmaceutical company, in 1974 with two employees and gradually turned it into a generic drug giant, eventually becoming the 15th richest person in Canada. Along the way, he faced legal action from family members alleging they'd been cut out of the company over the years.

Honey was a member of several boards for various organizations, including the York University Foundation and Mount Sinai's Women's Auxiliary.

Together, they were two of Canada's most generous philanthropists. Their children have established a charitable foundation named after their parents to continue their legacy.